Naive builders, particular on Windows without git installed,
would get builds calling themselves 1.0.1 even though master
has diverged significantly from the 1.0.x series at this point.

We should update this file both before and after release.

Previously we defined the release version string in configure.ac,
and overrode that with 'git describe --tags' if possible. This
made it difficult for non-autoconf builds to set their version
string correctly.

Instead we create, and check into version control, a file called
version.mk which defines OPUS_VERSION. The configure script reads
that file and uses it as a fallback if the git revision isn't available.

The expectation is that version.mk will be manually updated for
releases, just as the previous configure.ac version was. However,
since this is a simpler format, it is easier for alternate build
systems to use, reducing the number of places which must be updated.

Also removes the OPUS_MINOR_VERSION, etc. defines from config.h.